The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Traction drives utilize frictional force to transmit torque and power. Because the power is transmitted between two smooth surfaces, often through a thin layer of lubricant, a traction drive possesses unique characteristics that are not readily attainable by gear drives. The desired performance characteristics include quietness, high-efficiency, high rotational accuracy and zero-backlash.
According to the speed ratio change, traction drives can be classified into variable ratio drives and fixed ratio drives. Fixed ratio drives are often used as speed step-down or speed step-up units in various applications. The designs of the fixed ratio traction drives are in general simpler and the performance is more rugged than the variable ratio traction drives. The unique performance features, alone with inherit low manufacturing cost, have rendered traction drives a host of potential applications.
Generating adequate normal force at the frictional contacts is essential for traction drive transmissions. Planetary configurations are perhaps the most preferred arrangements for fixed ratio traction drives because the internal forces are often self-balanced. Various concepts have been proposed in the past to provide loading mechanisms.
Eccentric cylindrical planetary design offers one of the most effective means to generated torque-actuated normal force. By offsetting the outer ring eccentric to the sun roller, a convergent wedge gap is created along the annular space between the outer ring and the sun roller. When a planetary roller is in the converged wedge gap, frictional forces at the contacts tend to pull the planetary roller towards the smaller end of the gap, wedging the roller against the outer ring and the sun roller. An appreciable amount of normal load is thus generated at the frictional contacts. The normal load is in direct proportion to the applied torque load. The wedge loading concept was disclosed by Dieterch in U.S. Pat. No. 1,093,922, and was improved by others including Nelson in U.S. Pat. No. 3,945,270 and Ai in U.S. Pat. No. 6,981,930.
For practical considerations, the eccentric cylindrical designs disclosed in the prior art, were confined mostly to a speed ratio, defined as raceway diametrical ratio of the outer ring to the sun roller, less than 8:1. As the speed ratio increases, the effective contact radius at the contact between the planetary roller and sun roller becomes undesirable. This leads to a significant increase in contact stress and reduction in service life.
Therefore, it is highly desirable to provide a improved design that maintains a torque actuated loading mechanism and appreciably extends the speed ratio beyond the current practice with eccentric cylindrical traction drive